This invention relates to a microelectromechanical systems (MEMS) device and its method of manufacture. More particularly, this invention relates to a material and process for forming a MEMS electrical switch on a substrate.
Microelectromechanical systems (MEMS) are very small moveable structures made on a substrate using lithographic processing techniques, such as those used to manufacture semiconductor devices. MEMS devices may be moveable actuators, sensors, valves, pistons, or switches, for example, with characteristic dimensions of a few microns to hundreds of microns. A moveable MEMS switch, for example, may be used to connect one or more input terminals to one or more output terminals, all microfabricated on a substrate. The actuation means for the moveable switch may be thermal, piezoelectric, electrostatic, or magnetic, for example.
A thermal electrical switch may be formed, for example, by placing a conductive circuit adjacent to an elastically deformable flexor beam, and heating the conductive circuit by driving a current through it. The conductive circuit may be tethered to the elastically deformable flexor beam by a dielectric tether, such that the current does not flow to the deformable flexor beam from the conductive circuit. The conductive circuit may heat from Joule heating and expand relative to the deformable flexor beam, thus deflecting the deformable flexor beam to which it is tethered. If the elastically deformable flexor beam is coupled to an input terminal carrying an electrical signal, energizing the conductive circuit may deflect the elastically deformable flexor beam to a position in which it is in contact with another terminal, thereby connecting an input terminal to an output terminal and closing a switch. The conductive circuit and deformable component may therefore constitute an electrical switch. Such a switch may be used in, for example, telecommunications applications.
Because the thermal MEMS switch uses electricity and transmits an electrical signal, its various components may be made from conductive materials having specific electrical attributes. In particular, the conductive circuit of this thermal switch may need to have a finite resistivity, in order to carry the current, and be heated by the current. However, the elastically deformable flexor beam needs low resistivity, in order to carry the electrical signal without a large loss. However, in order to simplify the manufacture of such a switch, they are typically made out of the same material, and deposited in the same process step. As a result, the material forming the conductive circuit may not be optimized for its intended function, and the material forming the passive component may also not be optimized for its intended function.
Among the mechanical properties desired for the deformable flexor beam are high elasticity, such that the deformable flexor beam returns to its original position after deflection by the conductive circuit, high yield strength so that it is not permanently deformed by the deflection. Therefore, in addition to the low resistance needed by the flexor beam, it may also be advantageous to use a material with high elasticity and high strength. Among the characteristics desired for the conductive circuit are relatively low strength and somewhat higher resistance. Nonetheless, pure nickel is typically used for both the passive flexor beam as well as for the conductive circuit because of its generally acceptable resistance values and ease of well-known deposition processes, such as electroplating. However, nickel has relatively poor creep and strength characteristics, such that it is far from an ideal material from which to fabricate the switch components, particularly the flexor beam.